Fibroblast growth factors (FGFs) execute their ubiquitous roles in the developing embryo, as well as in the adult, by binding and activating FGF receptor tyrosine kinases (FGFRs). FGFRs are single pass transmembrane receptors composed of an extracellular ligand binding region and a cytoplasmic region harboring the conserved tyrosine kinase domain. FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is determined by primary sequence differences among FGFs and FGFRs. Similarly, specific recognition and tyrosine phosphorylation of intracellular targets by the activated FGFR is a fundamental step in FGF signaling and determines which specific downstream pathways are activated and, hence, what cellular response ensues. Aberrant FGF signaling is responsible for a wide spectrum of human pathological conditions including skeletal syndromes, olfactory syndromes, phosphate wasting disorders and cancer. The diversity of these diseases reflects the versatile and vital functions that FGFs play in human biology and provides a strong impetus for a thorough understanding of FGF signaling at the molecular level. The specific aims of this proposal are: I. Establish the pattern of, and determine the structural basis for, FGF-FGFR binding specificity/promiscuity. II. Elucidate the structural basis for autoinhibition in the extracellular region of FGFR. III. Elucidate the structural basis by which FGFR interacts with intracellular signaling molecules. IV. Elucidate the structural basis by which FGFR kinase domain mutations result in constitutive activation of FGFRs in human skeletal syndromes and cancer. The primary means to accomplish these aims will be X-ray crystallography, coupled with surface plasmon resonance and steady-state kinetic analysis. The fundamental structural and biochemical information obtained from these studies will enhance our knowledge of FGF signaling and will allow us to understand the effects of pathogenic FGF and FGFR mutations. In broader terms, these studies will facilitate the rational design of novel antagonists of FGF signaling for use in treatment of a variety of pathological conditions and will also enhance our understanding of signaling of the entire receptor tyrosine kinase superfamily.